1. Field of the Invention
The present invention relates to a method for converting a resolution of a document to be transmitted at a high rate and which meets the standard resolution requirements of a facsimile apparatus of a G4 type, and an apparatus thereof.
2. Description of the Prior Art
After a modern facsimile apparatus has been invented by using a phototube of the vacuum type in 1925, the CCITT (International Telegraph and Telephone Consulting Committee of the International Telecommunications Union) recommended a facsimile apparatus of a G1 type in 1968. The G1 type facsimile apparatus spends about six minutes in transmitting a document which corresponds to one page of A4-sized paper. According to a G2 standard which was established in 1976, it takes less than three minutes to transmit a document corresponding to one page of A4-sized paper via the facsimile apparatus.
A G3 type facsimile apparatus which is widely used at present, was recommended in 1980. The G3 type facsimile apparatus can transmit a document corresponding to one page of A4-sized paper via ordinary telephone lines within about one minute. The G3 type facsimile apparatus reduces the document transmission time by using a method for reducing information.
On the other hand, a G4 type facsimile apparatus connects with the ISDN (Integrated Services Digital Network) as a communication terminal equipment. When compared with the G3 type facsimile apparatus, the G4 type facsimile apparatus can elevate the resolution of the transmitted document by up to four times, and can raise the transmission rate by more than seven times (up to twenty times). Also, since the G4 type facsimile apparatus connects with the ISDN, it can offer composite and various communication services that the next generation of information communication requires.
When the print resolution of a receiving facsimile terminal is different from that of an originating facsimile terminal, in order to adjust the different print resolutions, a letter of advice relating to G4 standard of the ITU (International Telecommunication Union) specifies four resolutions such as 200, 240, 300 and 400 dots per inch! for the resolution conversion of the facsimile apparatus. The apparatuses for converting the resolution are embodied by means of software or hardware. However, the resolution converting apparatuses which are embodied by means of software, spend a lot of time to process.
For example, in the U.S. Pat. No. 4,876,604 granted to Hiroshi Nobuta, since the structure having image resolution converting circuits arranged at several positions in order to communicate between all the combinations of apparatuses which operate with different image resolutions, is complicated and costly, there is disclosed an image communication apparatus which has a memory for storing image data and which comprises image data supply means for supplying first and second image data of different resolutions, respectively, and an image data resolution converter connected between the image data supply means and the memory to convert the resolution of at least one of the first and second image data such that the resolution of the first and second image data supplied to the memory is unified.
The conventional facsimile apparatus as described above supports reduction/magnification ratio, e.g., 1/2-2 or 1/8-8 and not only performs all of the conversion functions with all of the magnifying powers, but also has the four kinds of resolutions specified in the letter of advice relating to G4 standard of the ITU. However, when the resolution converting apparatuses are embodied by means of hardware, the configuration thereof is complicated. When having three kinds of conversion functions for reduction, e.g., A3.fwdarw.A4, A3.fwdarw.B4 and B4.fwdarw.A4, the conventional facsimile apparatuses support only the reduction conversion mode which is frequently used, so the embodiment of the resolution converting apparatuses is simplified. However, the conventional facsimile apparatuses do not completely meet the G4 standard of the ITU.